The invention is based on a fuel injection device for internal combustion engines. In fuel injection devices of this type, the injection quantity is controlled either by means of control edges or via a magnet valve. In the case of the end of supply determined by the control edge or by opening of the magnet valve, a particularly problematic feature is that the valve needle does not close immediately, because the pressure cannot be reduced fast enough, because the line lengths and cross sections, and/or because the valve needle rises from its seat again because of reverse pressure waves in the lines. This has the disadvantage that fuel continues to be injected after the cutoff operation has already been completed. The result is increased harmful exhaust emissions. Furthermore, exhaust gases may get from the combustion chamber into the injection nozzle, which rapidly plugs the nozzle openings with carbon and causes deposits of soot particles.
The closing speed and closing quality of the injection valve needle can be increased, in a known manner, by increasing the closing pressure on the injection valve needle at the end of injection.
For instance, in a known injection device (German Patent No. 879 936), the valve closing spring is supported on its end remote from the nozzle needle on a piston that is acted upon on the other end with fuel from the pressure line via a fuel relief line provided with check valves disposed in contrary directions; this fuel simultaneously acts upon the valve needle in the opening direction. During the pump piston compression stroke, the closing spring of the valve needle is as a result more markedly pre-stressed via this piston, which leads to an increase in the closing pressure even during the injection process.
In another known injection device (U.S. Pat. No. 3,115,304) having a control edge for fuel quantity metering, the relief line is opened simultaneously with a diversion conduit of the pump work chamber. In the diversion or cutoff, the diverted fuel is directed via the relief line into the spring chamber, which increases the closing pressure. To assure a pressure reduction in the pressure chamber, the pressure valve has a valve plate with a throttle bore, for damping the diversion pressure, which because of the throttle bore does not assure adequate sealing of the pressure chamber with respect to the pump work chamber during the intake stroke.
In another known injection device (U.S. Pat. No. 3,075,707), which however belongs to the same generic type and likewise functions mechanically, the attempt is made to overcome this deficiency by using a plate valve, which during the compression stroke of the pump piston closes a relief line between the pump work chamber and the closing spring chamber, and upon cutoff or the intake stroke of the pump piston uncovers this relief line or its inlet opening to the spring chamber and closes it with respect to the pump work chamber.
Normally, a pressure valve serves to decouple the pump work chamber from the pressure chamber of the injection valve hydraulically during the intake stroke, to avoid undesirable pressure influence. Additionally, in electrically controlled injection pumps, this also prevents any existing pressure in the pressure line after the closure of the valve needle from recoiling on the magnet valve, which could cause its destruction or at least could cause inaccurate metering.
In each case, it is attained that a high injection pressure already prevails at the injection onset, which is good for the atomization or preparation of the fuel. Furthermore, it is attained that the pressure chamber of the valve needle is disconnected from the work chamber of the pump piston in the intake stroke, so that no negative pressure is produced in the pressure chamber from the intake stroke, which would have a deleterious effect on the opening characteristic of the valve needle in the ensuing compression stroke. In the provisions of the known injection device, where this reversal and the sealing off of the pump work chamber from both the pressure chamber and the valve closing spring chamber represents a compromise, the disadvantages noted initially above are still not overcome, because a clear separation between the functions of the pressure valve and the relief valve is not possible. During the switchover of the plate valve, uncontrollable leakage flows arise, which prevent accurate fuel quantity metering. A further disadvantage of this injection device is that with the relief line opened and fuel flowing out, a negative pressure is produced at the plate valve by the flow, and this impairs the sealing of the pump work chamber.